Audubon, Jan 1999 v101 i1 p22(1) 

The Ghostly Green Light. (what causes the northern lights) ANN ZWINGER. 

Abstract: Numerous myths and legends have been passed on to explain the northern lights, or
aurora borealis. The actual explanation for the spectacular phenomenon is that when solar
winds travelling 3 million miles per hour carry a stream of plasma into the earth's magnetic
field, electricity is generated, giving off nitrogen that pulses pink, green or red light into the
atmosphere. 

Full Text: COPYRIGHT 1999 National Audubon Society 

The earth's magnetic field and the solar wind combine to produce the heavenly show known as
the northern lights. 

I DON'T KNOW HOW OLD I was, but I'd certainly heard the phrase "the end of the world," and
that's exactly what I thought I was seeing when my mother called me outside one summer
evening in northern Michigan to see a display of the aurora borealis. Above me, a terrifying,
restless mass of eerie lights--wavering and whipping, dancing and writhing--pulsated across
the sky. The solid upside-down-bowl of night had fractured into an unreliable, quivering
universe. 

My mother's explanation that the name came from Aurora, the Greco-Roman goddess of dawn,
and that borealis meant "north" did nothing to calm me. I was a child caught in a dread
realization: Dazzling beauty can also contain terror. It was fear I saw, not dawn. 

My reaction couldn't have been much different from that of people who, centuries earlier,
dreamed up stories and myths to explain the northern lights. Greenlanders believed that their
island was perched on the outer edge of the world, and they conjectured that the aurora
borealis was a ring of fire surrounding the earth, or glaciers full of powerful spirits spewing
flames. Medieval people, after what must have been a period of extraordinary sunspot activity
that suffused the skies with red, imagined heads separated from bodies by bloody swords, and
undertook pilgrimages to save themselves from disaster. One early explorer referred to the
aurora borealis as "angel light," a manifestation of God's power. The Finns saw "fire foxes"; the
Scots, "merry dancers." 

Nontravelers living in latitudes lower than about 60 degrees could envision the colorful
northern lights only by reading the journals of Arctic explorers or by looking at black-and-white
engravings made from the sketches of shipboard artists--or perhaps by viewing the more skillful
woodcuts of the Norwegian explorer Fridjof Nansen, who in 1893 let his research vessel become
frozen into the ice so he could observe the aurora. 

Although black-and-white photographs of the northern lights have been taken since 1900, their
quality was unremarkable; not until fairly recently have accurate, full-color portrayals of the
fast-moving auroras been available, made possible by modern still and video cameras. And
move the auroras do: One, 60 miles up, was reported as being 3,000 miles wide and 100 miles
tall, galloping southward at 700 miles an hour. 

For observers who can enjoy the northern lights firsthand, the auroral display is an awesome
show that develops predictably over two or three hours. Given the right atmospheric conditions,
the auroras can be seen at any time of the year. What an observer sees varies according to
latitude. (The closer to the North Pole you are, of course, the better you can see the northern
lights; on rare occasions, though, you can see them from as far south as the Caribbean.) As night
begins, pale ribbons of light lie low toward the horizon. As they intensify in brilliance, they
coalesce to form a curtain that elaborates into complex rays and waves as it begins to surge
westward. By midnight, patches and shreds of curtain fill the sky before the lights dissolve into
tatters of luminosity and dawn swallows up the fabric that will be rewoven with endless
variation in the next cycle. 

Watchers have tried over the centuries to categorize the various threads: Forms constantly
segue into one another, flickering into arcs and rays; bands, bolts, or streaks; ribbons or
curtains, sometimes pleated, sometimes overlapping; and glows that, 60 to 250 miles above the
earth, suffuse the sky with red. 

Attempts to understand the spectacle have long provoked questions about how auroras form and
what powers these visions in the night sky. In the sixth century B.C., Hippocrates thought that
the sun passed under the earth to provide the darkness of night but that some rays escaped, slid
past the earth's edge, and lit vapors to create glimmering clouds. Aristotle believed that the
heavens were stable and that the northern lights were exhalations of the earth--an idea as
lovely as it was wrong. After many equally ingenious and erroneous proposals, Kristian
Birkeland, a professor of physics at the University of Oslo, suggested that electromagnetism
underlay the phenomenon. When he died, in 1917, his ideas were unaccepted by the scientific
community, but he had hinted at the right direction. Scientists finally recognized that
electricity was involved, and they considered harnessing it to help, in the words of a
19th-century reporter, "develop the brains of our statesmen and legislators, to make them wiser
and better and of more practical use than they are at present." 

The northern lights, a stunning occurrence so attentively observed for so many years, have
been accurately explained only within the past few decades. The defining studies began in the
International Geophysical Year of 1957-1958, and they were timed as part of a worldwide
research program during a period of maximum sunspot activity--electromagnetic disturbances
on the surface of the sun. Researchers in that effort employed the first "all-sky cameras," which
are capable of photographing the sky one hemisphere at a time. The initial observations from
space were made from the high-flying satellite Explorer I. The first space photographs, taken by
ISIS I, verified that the auroras existed simultaneously at both the North Pole and the South
Pole. (Captain James Cook, on his voyage to Antarctica at the end of the 18th century, first
reported the southern lights--aurora australis. They are still less known and less written about
than their northern counterpart, since much of their activity occurs over unpopulated areas.) 

Discoveries by satellite have continued over the years: In 1962 Mariner 2 measured the solar
wind, and in 1994 and 1995 Ulysses clocked it at 500 miles a second. In 1983 HILAT--the High
Latitude Satellite--took images of the complete oval display. In 1989 the Japanese launched a
series of satellites to make further detailed studies of auroras. 

Astronauts continue to be privy to sensational views of the auroras. They look down at what
Syun-Ichi Akasofu, the director of the Geophysical Institute at the University of Alaska, in
Fairbanks, describes as "an electrical discharge powered by the solar magnetosphere
generator." 

Any generator must have two components--an electrical conductor and a magnetic field. To
generate electrical power, the conductor must move across the field to produce an
electromotive force. In the case of auroras, the conductor is the solar wind roaring through
space as fast as 3 million miles an hour, blasting out at 100,000 degrees Celsius and carrying a
stream of plasma--a spray of charged particles, mostly hydrogen ions and electrons, that is
electrically neutral. 

The magnetic field that surrounds the earth is somewhat like a Japanese lantern surrounding a
lightbulb. The magnetic field is not spherical but is dimpled at each pole, like a navel, and it
protects us from being frizzled by solar radiation. Imagine fast-flowing water hitting a round
boulder: The stream flows around either side of the boulder and tails out downstream. That's
what happens when the streaming solar wind meets the boulder of the earth's magnetic field: It
flows out and around to form a wind-sock-shaped tunnel--the magnetosphere, which stretches
out in space far beyond the moon. All along the boundary where the solar wind and the
magnetic field meet, electricity is generated; the solar wind-magnetosphere generator pumps
out 9 billion kilowatt-hours annually--10 times the annual power production of the United
States. 

The solar wind shapes the magnetosphere the same way it shapes a comet's tail--remember
Hale-Bopp swishing across the sky with its tail always pointing away from the sun? The heat of
the sun ionizes and charges the particles in the solar wind; these particles collide with
upper-atmospheric atoms (mostly atomic oxygen and molecular nitrogen), which gain
energy--become "excited." When these atmospheric atoms return to their ground state, they
release energy in the form of light, each in its own characteristic set of colors. Since the auroras
form rings around both poles where the electric current from the solar wind-magnetosphere
generator enters and leaves the upper atmosphere, it is here that most collisions occur. The
light from these collisions appears as thin glowing curtains; from space, rings around the poles
are a footprint of the shape of the magnetosphere. 

Oxygen gives off what the Antarctic explorer Robert C. Scott called "the ghostly green light";
nitrogen molecules radiate pink. Atomic oxygen, Scott said, can also pulse out a dark "bloody
red color"; observers still remember a display of auroral red seen as far south as Florida in 1988.
(Displays visible at lower latitudes tend to be red.) The unusually intense displays of 1988
occurred at the onset of a period of maximum sunspot activity; for reasons that are imperfectly
understood, a peak in sunspot activity takes place about every 11 years. 

The northern lights arrive in pulses of greater or lesser activity. The solar wind originates in
"coronal holes," areas empty of sunspots; the sun's rotation is about 27 days, so a coronal hole
swings toward the earth at that interval. When that happens, matching visible auroral activity
increases. Large geomagnetic storms appear to occur every four to six weeks; intense storms
frequently disrupt such accoutrements of civilization as radio and navigational aids, and they
may even induce voltages in Arctic pipelines. They can also render satellites useless, as
happened in January 1997 to a $200 million AT&T satellite. Viewers in more southerly
locations, such as Hawaii and Mexico and Florida, report seeing the northern lights when the
solar wind bulges the auroral ovals toward the equator. 

As long as records of the northern lights have been kept, observers have reported a wide variety
of noises--from crackling and popping to swishing and whistling. Upon hearing any of these,
one may engage the aurora in conversation if one knows the proper technique. One Eskimo
myth claims that if you are out alone at night and hear the northern lights whistle, they will
come closer, possibly out of curiosity, if you whistle back. 

Akasofu disagrees; if there is sound, about which he is dubious, it would be of a frequency too
low for the human ear to hear. And the German geographer Alexander von Humboldt noted
more than a century ago that the "northern lights appear to have become less noisy since their
occurrences have been more accurately recorded." 

Even with the knowledge I have now, my mind still prickles with irrational sensations when I
see the mysterious magnetic blaze of ethereal beauty efflorescing across the night sky. And if
this is the way the world is going to end, so be it. 

ANN ZWINGER 

As a child, Ann Zwinger spent summers in northern Michigan. It was there that she
encountered the aurora borealis. She was terrified by the phenomenon, but she faced her fear in
writing this piece for Audubon. "That's the beauty of nature writing," she says. "I've confronted
the northern lights and learned about them, and now they're not so spooky." Zwinger has
explored natural wonders in 17 books and more than 30 magazine articles. She has also
illustrated several books. 

Ann Zwinger's most recent book is The Nearsighted Naturalist.